Default value for background_image_thumb fails when background_image URL includes a percent sign

[cfinke]

If the background_image theme mod is set to a URL that includes a percent sign, and if background_image_thumb is not set, the code that uses background_image as the default setting for background_image_thumb fails due to get_theme_mod interpreting the percent signs in the background_image URL as sprintf conversion specifications.

I don't think that this situation can occur in a vanilla WordPress install (but that might be host-specific), but it's entirely possible that a plugin would set background_image to a URL including a percent sign.

[cfinke]

Replaces single percent signs with double percent signs in anticipation of the sprintf call in get_theme_mod.

[SergeyBiryukov]

It's an issue with uploading any files containing percent signs in their names, not just with custom background. Going to close as a duplicate of #16330, since the patch there resolves this as well.

[cfinke]

I disagree that this is a duplicate of #16330. That bug concerns sanitizing the filenames of uploaded media; this bug concerns passing a string through sprintf() without considering that it could include strings that look like conversion specifications but aren't.

None of the fixes being discussed in #16330 would fix the bug presented by a plugin setting background_image to a URL like ​http://example.com/some%2Fbackground%2Fimage%2Furl.png.

[SergeyBiryukov]

Indeed, I missed the sprintf() part. Thanks for the clarification.

[nacin]

This is goofy. At first I thought this should be added to get_theme_mod(), but then I realized that this is specifically the *default* value, which by design could take %s, so that's wrong. Then I thought that get_background_image() could return %s, but that's *also* wrong because it too goes through get_theme_mod(). Just wanted to describe my thought process before committing.

[nacin]

In 24630:

When generating a background image thumbnail URL, escape percent signs for the eventual sprintf() call inside get_theme_mod().

props cfinke.
fixes #21241.